2.1 The Complement System
The complement system is a group of proteins that constitutes about 10 percent of the globulins in the normal serum of humans (Hood, L.E. et al. 1984, Immunology, 2d Edition, The Benjamin/Cummings Publishing Co., Menlo Park, Calif., p. 339). Complement (C) plays an important role in the mediation of immune and allergic reactions (Rapp, H. J. and Borsos, T., 1970, Molecular Basis of Complement Action, Appleton-Century-Crofts (Meredith), New York). The activation of C components leads to the generation of a group of factors, including chemotactic peptides that mediate the inflammation associated with complement-dependent diseases. The sequential activation of the complement cascade may occur via the classical pathway involving antigen-antibody complexes, or by an alternative pathway which involves the recognition of certain cell wall polysaccharides. The activities mediated by activated complement proteins include lysis of target cells, chemotaxis, opsonization, stimulation of vascular and other smooth muscle cells, degranulation of mast cells, increased permeability of small blood vessels, directed migration of leukocytes, and activation of B lymphocytes, macrophages and neutrophils (Eisen, H.N., 1974, Immunology, Harper & Row, Publishers, Inc., Hagerstown, Md., p. 512).
During proteolytic cascade steps, biologically active peptide fragments, the anaphylatoxins C3a, C4a, and C5a (See WHO Scientific Group, WHO Tech. Rep. Ser. 1977, 606, 5 and references cited therein), are released fromthe third (C3), fourth (C4), and fifth (C5) native complement components (Hugli, T. E. CRC Crit. Rev. Immunol. 1981, 1, 321; Bult, H. and Herman, A. G. Agents Actions 1983, 13, 405). The C5a fragment, a cationic peptide derived from the first 74 amino acids of the amino-terminus of the C5 alpha subunit (Tack, B.F. et al. Biochemistry 1979, 18, 1490), is of particular pathological relevance. Regulation of C5a activity is by the endogenous plasma enzyme carboxypeptidase N (E.C. 3.4.12.7), which rapidly removes the carboxy-terminal arginine from C5a, producing the less potent but still active C5a des Arg. Reported effects of C3a and C5a upon specific immune responses are listed in Table I.
TABLE I ______________________________________ EFFECTS OF COMPLEMENT COMPONENTS C3a AND C5a ON SPECIFIC IMMUNE RESPONSES Immune Response C3a C5a/C5a des Arg ______________________________________ Specific antibody production Suppression Enhancement in response to sheep red blood cells Polyclonal antibody Suppression Enhancement production in response to Fc antibody fragment T cell proliferation Suppression Enhancement in response to tetanous toxoid T cell proliferation in No effect Enhancement mixed lymphocyte reaction T cell-mediated cytotoxicity Suppression Enhancement ______________________________________
Among the wide variety of biological activities exhibited by C5a are contraction of smooth muscle (Wissler, J. H. Eur. J. Immunol. 1972, 2, 73), degranulation of mast cells (Johnson, A. R. et al., Immunol. 1975, 28, 1067), secretion of azurophilic granular enzymes from polymorphonuclear neutrophils (PMN) (Webster, R. O. et al., Immunopharmacol. 1980, 2, 201), and the chemotaxis of PMN (Wissler, J. H. Eur. J. Immunol. 1972, 2, 73; Becker, E. L. Trends Pharmacol. Sci. 1983, 4, 223) (Table II).
TABLE II ______________________________________ BIOLOGICAL EFFECTS OF C5a ______________________________________ I. Stimulation of neutrophil functions involved in inflammation A. chemotaxis B. chemokinesis C. aggregation D. lysosomal enzyme release E. generation of toxic oxygen products II. Smooth muscle effects A. stomach smooth muscle contraction B. vasodilation II. Promotion of histamine release A. mast cells B. basophils IV. Immunoregulatory effects ______________________________________
The active chemotactic factor in vivo is considered to be C5a des Arg (Becker, E.L. Trends Pharmacol. Sci. 1983, 4, 223).
The C5a or C5a des Arg fragments have been implicated in the infiltration of PMN (the chemotactic effect) in rheumatoid arthritis, certain forms of glomerulonephritis, experimental vasculitides such as the Arthus reaction, the acute pneumonitis produced by the instillation of chemotactic factors into the lungs of experimental animals with resulting release of leukotrienes C-4 and D-4 (LTC.sub.4 and LTD.sub.4), etc. In addition, the interactions between C5a and neutrophils have been considered to underlie tissue damage in several clinical situations. For instance, there exists a growing body of evidence for the role of oxygen-derived free radicals in mediating myocardial tissue injury during myocardial ischemia and, in particular, during the phase of myocardial reoxygenation and reperfusion. Among a number of possible sources of these radicals, the polymorphonuclear neutrophil has been the focus of primary attention. Studies have documented that neutrophil depletion or suppression of neutrophil function results in a significant salvage of myocardial tissue that is subjected to a period of regional ischemia followed by reperfusion (Simpson, P. J. and Lucchesi, B. R. J. Lab. Clin. Med. 1987, 110(1), 13-30). Neutrophil depletion in dogs resulted in significantly smaller myocardial infarcts after 90 minute occlusion with 24 hour reperfusion (Jolly, S. R. et al. Am. Heart J. 1986, 112, 682-690).
One study documented the activation of complement and generation of oxygen-derived free radicals during cardiopulmonary bypass. The administration of protamine during cardiopulmonary bypass further activated complement (Cavarocchi, N. C. et al., Circulation 1986, 74, 130-133; Kirklen, J. K. et al. J. Thorac. Cardiovasc. Surg. 1983, 86, 845-857). Also, recombinant tissue plasminogen activator (r-TPA), which in recent clinical trials has been found to be an effective thrombolytic agent in patients with acute myocardial infarction, was shown to activate complement. A striking increase in the level of C4a, C3a, and C5a was found in patients receiving r-TPA as compared to the level of these complement peptides before administration of the drug (Bennett, W. R. et al. J. Am. Coll. Cardiol. 1987, 10(3), 627-632). Schafer and co-workers were able to positively identify the deposition of terminal C5b-9 complement complex in myocardial cells located within zones of infarction in human tissue (J. Immunol. 1986, 137(6), 1945-1949). Likewise, the selective accumulation of the first component of complement and leukocytes in ischemic canine heart muscle has been found (Rosen, R. D. et al. Circ. Research, 1985, 57, 119-230). In one study, the depletion of complement was found to increase the blood flow in ischemic canine myocardium. This increased blood flow was found, in turn, to increase the supply and utilization of oxygen in complement depleted animals versus control animals (Grover, G. J. and Weiss, H. R. Basic Res. Cardio. 1987, 82(1), 57-65). Complement activation is also believed to initiate adult respiratory distress syndrome (ARDS). This syndrome, also known as adult respiratory failure, shock lung, diffuse alveolar damage, or traumatic wet lungs, is characterized clinically by the rapid onset of severe life-threatening respiratory insufficiency that is refractory to oxygen therapy. (Miescher, P. A. and Muller-Eberhard, H. J., eds., 1976, Text Book of Immunopathology, 2d Ed., Vols. I and II, Grune and Stratton, New York; Sandberg, A. L., 1981, in Cellular Functions in Immunity and Inflammation, Oppenheim, J. J. et al., eds. Elsevier/North Holland, N.Y., p. 373; Conrow, R.B. et al. J. Med. Chem 1980, 23, 242; Regal, J. F.; and Pickering, R. H. Int. J. Immunopharmacol. 1983, 104, 617). Some of the clinical implications of C5a release are listed in Table III.
TABLE III ______________________________________ CLINICAL IMPLICATIONS OF C5a RELEASE ______________________________________ Rheumatoid Arthritis Acute Gouty Arthritis Acute Immunological Arthritis Pulmonary Disorders Adult Respiratory Distress Syndrome Pulmonary Dysfunction - Hemodialysis Chronic Progressive Pulmonary Dis-Cystic Fibrosis Byssinosis Asbestos-Induced Inflammation Inflammation of Systemic Lupus Erythematosus Inflammation of Glomerulonephritis Purtscher's Retinopathy Hemorrhagic Pancreatitis Renal Cortical Necrosis Primary Biliary Cirrhosis Inflammation Nephropathology Cranial Nerve Damage in Meningitis Tumor Cell Metastasis Extended Tissue Destruction in Myocardial Infarction Extended Tissue Destruction in Burns ______________________________________